The present invention relates to a screw rotor-type dry vacuum pump and, more specifically, to a vacuum punp having a corrosion-resistance against gas generated in an apparatus for producing semiconductors, a dry vacuum pump in which a corrosion-resistant nickel alloy is employed as a material of a casing and a screw rotor that come in contact with a corrosive fluid, and a dry vacuum pump in which reaction products of process gas in an apparatus for producing semiconductors are prevented from building up in a path of a blast pipe of the dry vacuum pump.
A structure of a screw rotor-type dry vacuum pump will be explained with reference to its transverse sectional view shown in FIG. 1. A pump casing consists of: a main casing 1; an inlet-side case 2 attached to a right end face of the main casing 1; an outlet-side case 3 attached to a left end face of the main casing 1; and a gear case 4 attached to a left end face of the outlet-side case 3. A motor 5 is attached to the gear case 4.
In the main casing 1, there is provided an inner cylinder 1a penetrating through the main casing 1 axially, then an inlet 6 provided in the main casing 1 communicates with the right side of the inner cylinder 1a and then, the left side of the inner cylinder 1a communicates with an outlet 7 provided in the outlet-side case 3. An abbreviation numeral 8 denotes a chamber of cooling water.
Two through holes 9 are formed in the inlet-side case 2 and a bearing box 10 containing a bearing 11 therein is attached to each through hole 9. Two through holes 12 are formed in the outlet-side case 3 and a bearing box 13 containing a bearing 14 therein is attached to each through hole 12.
Each of two screw rotors 15 consists of: spiral toothlike parts 15a, a cross section of each of which is formed by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve; and a shaft 15b formed at both sides of each toothlike part 15a. The toothlike parts 15a are received in the inner cylinder 1a intermeshing with each other and each shaft 15b is supported by the bearing 11 or bearing 14.
As to the drive-side screw rotor 15 shown at the lower side in FIG. 1 out of the two screw rotors 15, a timing gear 16 is inserted into a left end of the shaft 15b, then fixed by a locking mechanism 17, while the left end of the shaft 15b is connected to an output shaft of the motor 5 through a coupling 18. As to the follower-side screw rotor 15 shown at the upper side in FIG. 1 out of the two screw rotors 15, a timing gear 19 that engages with the timing gear 16 is inserted into a left end of the shaft 15b, then fixed by the locking mechanism 17.
As shown in FIG. 2, i.e. a partially enlarged view of FIG. 1, the locking mechanism 17 consists of a locking member 20 and a tightening member 21, then a engaging portion 22 for engaging with an outer peripheral surface of the shaft 15b is formed at one face of the locking member 20, then a through hole 24 mating with a screw hole 23 formed on an end face of the shaft 15b is formed and then, a pushing projection 25 is formed outside the engaging portion 22. When the engaging portion 22 of the locking member 20 is inserted into the shaft 15b, the locking member 20 is firmly mounted to the shaft 15b and the pushing projection 25 abuts on a bottom of a circular groove 26 formed on a side of the timing gear 16.
The tightening member 21 is a bolt. When its end is screwed into the screw hole 23 through the through hole 24 of the locking member 20, the pushing projection 25 pushes the timing gear 16, then the timing gear 16 is pressed between the bearing 14 and the pushing projection 25 and fixed to the shaft 15b. 
When the motor 5 revolves, the coupling 18 and the drive-side screw rotor 15 revolve, then the revolution of the drive-side screw rotor 15 is transmitted to the follower-side screw rotor 15 through the timing gears 16 and 19, then the two screw rotors 15 revolve in an opposite direction with each other at the same speed so as to transfer the fluid pumped from the inlet 6 to the outlet 7. During this operation, a portion communicated with the inlet 6 is gradually depressed and the main casing 1 is heated, therefore, the main casing 1 is water-cooled.
As to a conventional vacuum pump for use in an apparatus for producing semiconductors, since corrosive gas is pumped up, a resin coating has been generally performed on surfaces of the inner cylinder 1a and the screw rotor 15. For example, Tefron coating or Defric (polyimide resin) coating has been performed on an inner surface of the inner cylinder 1a and a surface of the screw rotor 15 up to the thickness of 25 to 30 xcexcm.
Recently however, as to the apparatus for producing semiconductors, micro machining employing plasma has been widely used, then fluoride such as CF4 and C2F6 have been widely employed as to such apparatus for producing semiconductors in order to clean the apparatus during the manufacturing process. Above all, processes of a plasma-induced chemical vapour deposition and plasma etcher have been frequently employed, in which the fluoride such as CF4 and C2F6 is fed to remove products generated by nitriding, resulting in generation of activated fluorine system F* due to an excitaion by plasma. Since this F* is chemically very active, it reacts with H2 gas contained in a process gas to generate HF. This very corrosive HF gas corrodes the resin coating and pulverizes them. Above all, since a vacuum pump employed for the process involving the generation of the products generated by nitriding is heated in order to prevent the products from solidifying and piling up in a casing of the vacuum pump, the reaction of HF production is accelerated, resulting in peeling of the resin coating.
When the resin coating performed on an inner surface of the inner cylinder 1a and a surface of the screw rotor 15 up to the thickness of 25 to 30 xcexcm peels off, a gap having a diameter of 100 to 120 xcexcm is generated between the screw rotor 15 and the inner cylinder 1a, causing a severe deterioration in the performance of the vacuum pump. Since the dry vacuum pump does not use a sealing liquid, the enlargement of the gap brings about a serious defect.
As a measure for solving the problem mentioned above, a corrosion-resistant material might be employed for the screw rotor 15 and the main casing 1 without coating them, however, such a corrosion-resistant material, i.e. SUS (stainless steel) is very hard to be machined. Therefore, SUS is not appropriate for the screw rotor 15 that has a complex shape and requires highly dimensional accuracy. In addition, since SUS has a large coefficient of thermal expansion and a drawback that a seizure is easily occurred, SUS can not be employed as a material for the screw rotor 15 and the main casing 1.
A corrosion-resistant material, in which nickel is added to a spheroidal graphite cast iron having high mechanical strength, has been used to make the screw rotor 15 and the main casing 1. However, since its coefficient of thermal expansion depends on the added amount of nickel and is different from that of the locking mechanism 17 made of mild steel, the locking mechanism 17 becomes loose, causing a slip for the timing gears 16 and 19 and an undesirable contact between screw rotors 15 with each other.
In addition, a bearing fitting portion between the bearing 14 that supports the shaft 15b and the bearing box 13 often suffers a creep phenomenon and the bearing 14 often suffers a damage.
The present invention is to solve the above problems by making a spheroidal graphite cast iron containing nickel, which has the same coefficient of thermal expansion with that of the locking mechanism 17 made of mild steel, taking advantage that its coefficient of thermal expansion can be adjusted by varying the added amount of nickel.
As described above, when the output shaft of the motor 5 revolves, the drive-side screw rotor 15 revolves, then the follower-side screw rotor 15 revolves in an opposite direction at the same speed, then the toothlike parts 15a revolve intermeshing with each other within the inner cylinder 1a in the main casing 1, resulting in that the fluid pumped from the inlet 6 of the main casing 1 is transferred to the outlet 7 of the outlet-side case 3 (see FIG. 8). Here, since a temperature elevation at the outlet side of each toothlike part 15a is larger than that at the inlet side thereof, a tapered face of 1/(10 L) (L: length of the toothlike part 15a), which decreases in diameter toward the outlet side, is formed with respect to an outer diameter of each toothlike part 15a, by taking the thermal expansion of the outlet side into consideration.
Consequently, an outer diameter dimension D1 at the inlet side end of each toothlike part 15a is set so that a clearance of 0.2 to 0.25 mm in diameter can be formed against the inner diameter of the inner cylinder 1a of the main casing 1, while an outer diameter dimension D2 at the outlet side end of each toothlike part 15a is set so that a clearance of 0.3 to 0.35 mm in diameter can be formed against the inner diameter of the inner cylinder 1a of the main casing 1.
Although it is effective that the casing and the screw rotor of the dry vacuum pump are made of cast iron containing nickel, the following problems have arisen.
That is, such a material is corrosion-resistant, but has poor machinability. When the length of the inner cylinder 1a of the main casing 1 is long and five times as long as the inner diameter of the inner cylinder 1a, a deflection arises for a boring bar BB due to high cutting force upon boring machining of the inner cylinder 1a, resulting in a problem that a tool BT at an end of the boring bar BB veers away from the right direction (see FIG. 9).
The boring bar BB can be shortened by machining the inner face of the inner cylinder 1a of the main casing 1 from both sides by a length of 0.5 L each. However, in this case, the main casing 1 should be reset by turning it with 180xc2x0 in angle after a boring of one side by the length of 0.5 L is finished, resulting in that a discrepancy of 0.01 to 0.02 mm between central lines of two inner faces might arise after the machining.
If a small positional discrepancy arises for the central lines, the inner cylinder 1a easily comes in contact with an outer peripheral surface of each toothlike part 15a of the screw rotor 15 (see FIG. 10), as if an inner diameter of a central portion of the inner surface of the inner cylinder 1a becomes small by the same size of this discrepancy.
In addition, cast iron containing nickel has a larger coefficient of thermal expansuon in comparison with that of general cast iron, causing a problem that it deforms due to thermal strain at high temperature.
When a strain of casing arises due to heating of the casing during an operation of the pump, a seizure phenomenon arises at a sliding portion between the casing and the screw rotor. This problem of the seizure phenomenon has been hard to solve.
Various experiments have been tried to solve the above problem. Since the dry vacuum pump is required to have a performance that the degree of vacuum becomes 10xe2x88x923 Torr (i.e. order of 1 Pa) within 15 to 20 minutes after the start of operation, a measure that an outer diameter of the screw rotor is set small so as to enlarge the gap between the screw rotor 15 and the inner cylinder 1a makes no solution as to the above problem.
Moreover, when the resin coating on the outer face of the screw rotor is performed, the gap enlarges further due to the peeling of the resin coating having thickness of 20 to 30 xcexcm, causing a severe deterioration in the performance of the pump. Consequently, the method of resin coating needs some contrivance.
Through various experiments, we have studied a thermal expansion and thermal strain of the casing and the screw rotor made of cast iron containing nickel at elevated temperature and then, we have found the desirable gap, in which an amount of thermal expansion, an amount of deformation and the discrepancy of the central lines described above obtained by the present precision of machining are taken into consideration with respect to the portion from the vicinity of the center of the casing up to the outlet side thereof.
On the basis of the above experiments, the present invention is reached under the consideration of allowable dimensional accuracy for machining. The present invention is to provide a dry vacuum pump, in which the casing and the screw rotor are made of cast iron containing nickel that is hard to be machined and a seizure phenomenon never arises when the pump is heated during the operation, by securing the allowable dimensional accuracy determined through the above experiments.
In addition, the following distinct problem has been existed as to the dry vacuum pump.
As shown in FIG. 13, when two screw rotors 15 revolve due to a drive by the motor 5, the fluid pumped from the inlet 6 of the main casing 1 is transferred to the outlet 7 of the main casing 1, then passes through a silencer 31 while passing through an outlet path 30 that is connected to the outlet 7 and then, is discharged to a scrubber 32 from an end of the outlet path 30.
A dry vacuum pump, in which process gases are treated, is called a pump for use in hard process. Here, the process gas is used in an apparatus for producing semiconductors at low pressure and thin film nitrides are formed by the process using CVD (chemical vapour deposition) method and TEOS (tetraethoxysilane) AL Etcher.
A process gas flowing in the casing 1 of a dry vacuum pump A is highly compressed on its way to the outlet 7 (see FIG. 13), then AlCl3 and NH3Cl generated via the hard process are heated by the heat of compression and discharged from the outlet 7 without solidifying in the casing 1.
However, the process gas pumped at a pressure around 100 to 10xe2x88x923 Torr is a diluted gas having 10xe2x88x923 to 10xe2x88x926 of atmospheric pressure and has small heat capacity even at high temperature. Therefore, the process gas is easily cooled down in the outlet path 30 and the silencer 31, then products in the gas, which solidify due to the cooling, often close the outlet path, causing a tripping or a seizure phenomenon for the motor 5 of the dry vacuum pump A during the production of semiconductors and causing a severe loss in the production of semiconductors.
In order to prevent the products from solidifying, the diluted gas must be prevented from being cooled down in the outlet path 30. Therefore, the diluted gas is prevented from being cooled by attaching a heater 33 or a heat insulating material 34 to the outlet path 30. Instead, the outlet path 30 is frequently disassembled and cleaned to remove the products deposited there.
However, to employ the heater 33 is not appropriate from the viewpoint of preventing fire or saving energy. The cooling of the outlet 30 should be prevented from occurring without using the heater 33 in order to avoid a time-consuming disassembly and cleaning of the outlet 30.
It is therefore an objective of the present invention to solve the above problems and to provide a dry vacuum pump preventing the process gas from cooling down and having a structure, in which the products never deposited in the outlet path 30 of the dry vacuum pump.
In order to attain the above objective, a first aspect of the present invention is to provide a dry vacuum pump comprising: a casing having an inner cylinder communicating with an inlet and an outlet of the pump; a plurality of screw rotors, each of which comprises a shaft and spiral toothlike parts, received in the inner cylinder with the toothlike parts intermeshing with each other, said shaft is supported by the casing and said spiral toothlike part, a cross section of each of which is formed by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve, is formed integrally on the shaft; timing gears, each of which is attached to the respective shafts of the screw rotors, intermeshing with each other; and locking mechanisms, each of which is for fixing the timing gear to the shaft, wherein the screw rotor is made of spheroidal graphite cast iron containing nickel of 20 to 30% in weight and has substantially the same coefficient of thermal expansion with that of the locking mechanism made of mild steel.
The locking mechanism comprises: a locking member having an engaging portion for engaging to an outer peripheral surface of an end of the shaft and a pushing projection, an end of which abuts on the timing gear; and a tightening member for pressing the pushing projection onto the timing gear.
A second aspect of the present invention is to provide a dry vacuum pump characterized in that a screw rotor comprises: shafts, both ends of which are supported by a casing; and spiral toothlike parts, each of which is formed on an outer surface of the shaft except on both ends of the shaft, a cross section of the spiral toothlike part is formed asymmetrically spiral by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve, and a pair of the screw rotors rotates in an inner cylinder of the casing with the toothlike parts intermeshing with each other so that fluid in the casing is transferred from an inlet side to an outlet side of the pump, in addition, as for the rest, there are two kinds of invention as follows: (1) only the screw rotor is dimensionally adjusted; or (2) both of the screw rotor and the casing are dimensionally adjusted.
The above invention (1) is characterized in that a tapered face of 1/(20 L) is formed with respect to the toothlike part so that an outer diameter of the toothlike part is shortened from the center of the toothlike part to the outlet side of the fluid, L being a length of the toothlike part, and a ground finish-surface is formed with respect to the toothlike part so that a diameter of the toothlike part is shortened by 3/100 to 4/100 mm from a position, where is about 10 mm offset toward the inlet side from the center of the toothlike part, to the outlet side.
The above invention (2) is characterized in that a tapered face of 6/(100 L) to 7/(100 L) is formed with respect to the toothlike part so that an outer diameter of the toothlike part is shortened from the center of the toothlike part to the outlet side of the fluid, L being a length of the toothlike part, and an internal diameter of the inner cylinder is enlarged by 3/100 to 4/100 mm from a position, where is about 10 mm offset toward the inlet side from the center of the inner cylinder, to the outlet side.
A third aspect of the present invention is to provide a screw rotor-type dry vacuum pump characterized in that a pair of right and left handed screw rotors, a cross section of each of which is formed by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve, is received in a casing intermeshing with each other so that a process gas pumped from an inlet of the casing is discharged from an outlet of the casing, wherein the screw rotor has a plurality of leads, a nitrogen-supplying tube communicates with a position near the outlet in the casing, and an outlet path connecting the outlet with a scrubber or a trap is a straight pipe, in which a silencer is removed.
The dry vacuum pump is for use in a hard process, in which the dry vacuum pump pumps up a process gas employed in an apparatus for producing semiconductors.